WEDNESDAY, July 15, 2015 (HealthDay News) -- Among women with a family history of breast cancer, breast cancer survivors tend to gain more weight than women who are free of the disease, new research suggests.

And that added weight might increase the risk of heart disease and diabetes, as well as recurrence of the cancer, the researchers said.

The researchers compared 303 breast cancer survivors with 307 women who were cancer-free. All were participants in a study of women with a familial risk of breast and ovarian cancer. They included women with BRCA1 and BRCA2 gene mutations that can raise breast cancer risk.

"We found that breast cancer survivors, especially those with chemotherapy [treatment], gained more weight compared to cancer-free women," said lead researcher Amy Gross, a doctoral candidate at Johns Hopkins Bloomberg School of Public Health in Baltimore. The study was published July 15 in the journal Cancer Epidemiology, Biomarkers & Prevention.

Overall, breast cancer survivors gained an average of about 4 pounds more than their cancer-free counterparts in the first five years after diagnosis, Gross and her colleagues found. Those who were diagnosed with estrogen receptor-negative breast cancer -- which doesn't need estrogen to grow -- gained an average of 7 pounds more than the cancer-free women.

And those who had chemotherapy gained even more weight. "We found that the survivors who had received chemotherapy were twice as likely to have gained at least 11 pounds [compared to cancer-free women]," Gross said.

What is it about the chemotherapy? "The bottom line is, we actually do not know," said senior study author Dr. Kala Visvanathan, director of the Clinical Cancer Genetics and Prevention Service at the Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins.

The researchers also could not explain why those who had ER-negative cancers gained more weight.

But the weight gain the researchers saw may increase the risk of heart disease and diabetes, as well as recurrence of the cancer, they noted.

It's difficult to explain the findings, agreed Dr. Laura Kruper, chief of breast surgery service and director of the Cooper Finkel Women's Health Center at the City of Hope Comprehensive Cancer Center, in Duarte, Calif.

"It would seem that chemotherapy and other breast cancer treatments have a significant effect on metabolism in patients undergoing treatment for breast cancer," said Kruper, who was not involved with the study.

The findings suggest that women who are breast cancer survivors should have their weight monitored closely and be aware that they may tend to put on weight, the researchers said.

Kruper agreed that both doctors and patients need to pay closer attention to weight gain.

"It might mean increasing the amount of exercise to combat the potential change in metabolism. Or perhaps decreasing caloric consumption, understanding that with a cancer diagnosis and subsequent treatment, one might need to change previous eating habits," Kruper said.

While that might be difficult, keeping weight under control is wise, she said, since gaining weight after a cancer diagnosis can increase the risk of recurrence.